In a large bowl, whisk egg yolks, flour, 3 tbsp. zest, 2 cups ricotta and 3 tbsp. sugar. In another large bowl, beat egg whites until stiff; fold into batter. On griddle, cook pancakes in butter over medium until golden, 3 minutes per side. In medium bowl, whisk remaining ricotta and sugar. Top pancakes with ricotta mixture and remaining zest.